Methods for manufacturing photoelectrosynthetically active heterostructures

ABSTRACT

A photoelectrosynthetically active heterostructure (PAH) is manufactured by forming or providing cavities in an electrically insulating material; forming or providing an electrically conductive layer on a side of the electrically insulating material; depositing an electrocatalyst cathode layer in the cavities; depositing one or more layers of light-absorbing semiconductor material in the cavities; depositing an electrocatalyst anode layer in the cavities; removing the layer of electrically conductive metal; and forming a hydrogen permeable layer over the electrocatalyst cathode layer. The one or more layers of light-absorbing semiconductor material can form a p-n junction or Schottky junction. The PAH can be used in photoelectrosynthetic processes to produce desired products, such as reduction product (e.g., methane gas, methanol, or carbon monoxide) from carbon dioxide and liquid waste streams.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.13/676,901, filed Nov. 14, 2012, which claims the benefit of U.S.Provisional Patent Application No. 61/559,717, filed Nov. 14, 2011, thedisclosures of which are incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of micro-scale photovoltaic devices, moreparticularly photoelectrochemical/photoelectrosynthetic devices,processes and systems for utilizing solar energy to drive chemicalreactions.

2. Review of Technology

There is great interest in renewable energy generation in order toreplace conventional fossil fuels. This includes utilizing solar, windand biomass for producing fuels. Solar energy is particularlyinteresting because it is a fundamental renewable energy source that canbe used to continuously, noiselessly, and passively generate fuels oncean infrastructure for fuel production has been developed. Photovoltaicdevices consist of semiconductor materials that are capable of capturingphotons from solar irradiation and converting them into electricalenergy (i.e., a current having an electrical potential).

Solar cells are effectively used to provide power for satellites inspace operations and stand alone appliances that are not connected tothe electrical grid. However, connecting solar cells to a grid is farmore complicated because the direct current power produced by solarcells must be converted to AC power according to the power transmissionsystem into which they are to be integrated. Moreover, solar cellsrequire complex and relatively expensive wiring to interconnect a systemof solar cells together to provide electrical power to be input into thegrid. Corrosion, heat stress and internal short-circuits can reduce oreliminate the ability of a solar cell to generate power. In the case ofsolar panels, failure of a subsection may cause catastrophic andirreparable failure and require replacement of the entire panel.

Solar energy can also be used in photoelectrosynthetic processes toproduce desired products through photo-oxidation and photo-reduction ofchemical components in a feedstock. For example, U.S. Pat. No. 4,263,110to Meyerand et al. discloses the use of semiconductor “platelets”suspended in a reactor vessel containing aqueous hydrobromic acid toproduce hydrogen gas and bromine as products (see FIGS. 1A and 1E ofthis disclosure). The semiconductor “platelet” 100 shown in FIG. 1Aincludes an n-doped semiconductor material 101 and a p-dopedsemiconductor material 102. An edgewise insulation material 103 (e.g.,conventional epoxy resin) is shown broken away to expose ohmic contacts104.

U.S. Pat. No. 4,094,751 to Nozik discloses Schottky-type and p-njunction type photochemical diodes. As shown in FIG. 1B, a cross-sectionof an exemplary Schottky diode 110 is shown suspended in a reactionmatrix 118 of a reactor 111 and includes an appropriately dopedsemiconductor 112 (n- or p-type), an ohmic contact 113 adjacent to thesemiconductor, and a metallic contact or layer 114 adjacent to the ohmiccontact 113. The Schottky diode 110 further includes asemiconductor/matrix interface 115 and a metal/matrix interface 117.Absorption of light energy 116 by the semiconductor layer 112 createselectrons and holes (not shown). For n-type semiconductors, electronsmove across the ohmic contact 113 to the metallic layer 114, where theyare injected through the metal/matrix interface 117 into the reactantmatrix 118 to drive a reduction reaction (such as H₂ evolution). Holesare injected through the semiconductor/matrix interface 115 into thereactant matrix to promote an oxidation reaction (such as the formationof O₂ or H₂O₂). The charge flows are reversed for p-type semiconductors.

FIGS. 1C and 1D show exemplary p-n junction type photochemical diodesdisclosed in Nozik. As shown in cross-section in FIG. 1C, an exemplaryside-by-side p-n junction type photochemical diode 120 is configured toabsorb incident light 121 on one side and includes a p-typesemiconductor 123, which is provided with an ohmic contact 124, and ann-type semiconductor 125, which is provided with an ohmic contact 126.The two ohmic contacts 124, 126 are optionally connected through a metalcontact 127, which serves to act as a support for the side-by-side diode120, which is shown suspended in reaction matrix 128.

As alternatively shown in cross-section in FIG. 1D, an exemplary stackedp-n junction type photochemical diode 130 is configured to absorbincident light 131 on both sides and includes a p-type semiconductor133, which is provided with an ohmic contact 134, and an n-typesemiconductor 135, which is provided with an ohmic contact 136. The twoohmic contacts 134, 136 are shown connected through an optional metalcontact 137.

FIG. 1E illustrates an exemplary apparatus 140 for producing hydrogenand bromine from hydrobromic acid, as disclosed in Meyerand et al.Apparatus 140 includes platelet particles 141, a hydrobromic acidelectrolyte solution 144 flowing as indicated by arrows 145. The flow ofthe electrolyte solution 144 can be such that the platelet particles 141remain substantially suspended between the area defined by lower andupper screens 146 a, 146 b. Lower screen 146 a primarily provides aresting place for the platelets 141 during shutdown, while lower screen146 a and upper screen 146 b confine errant platelets 141 duringpositive and negative flow surges and turbulence. Screens 146 a, 146 bcan be optionally removed once system stability is attained. Radiantenergy 143 drives the formation of bromine 147, which settles to thebottom of the apparatus, and hydrogen gas 149, which bubbles to thesurface and is expelled through port 148. The hydrobromic acidelectrolyte solution 144 can be run through a monitoring station 150 andadditional electrolyte added as needed.

Notwithstanding the foregoing, commercially feasible production ofhydrogen, bromine and other products using semiconductor powered devicesremains elusive and no commercial system for producing chemicals of anykind using artificial solar photoelectrosynthesis has ever beendemonstrated. Accordingly, there remains a need to find commercially andtechnically feasible ways to utilize solar energy to drive usefulchemical reactions. There is also a need to produce hydrocarbon fuelsfrom renewable energy sources such as solar energy. There is also anongoing need to remove or sequester carbon dioxide from the atmosphere.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein are methods for producing desired chemical products,including hydrocarbons such as methane and other alkanes, synthesis gas(carbon monoxide and hydrogen), and methanol, from carbon dioxide andoxidizable reactant compounds in wastewater as a feedstock using solarenergy to drive at least a portion of the chemical reaction process(e.g., to produce hydrogen gas). Photoelectrosynthetic processes employphotoelectrosynthetically active heterostructures (PAHs) to absorbsunlight and transform the light energy into electrochemical potentialenergy, which converts reactants containing hydrogen atoms intoproducts, which react with carbon dioxide to form desired chemicalproducts. Also disclosed are photoelectrosynthetic and thermochemicalreaction systems used to produce methane gas from a hydrogen containingsource, PAHs, solar energy, and carbon dioxide, as well as improved PAHshaving increased stability and efficiency and methods of making suchPAHs.

The methods and systems may employ photoelectrosynthetically activeheterostructures (PAHs) that are similar to those known in the art orimproved PAHs disclosed herein which have improved stability, energyconversion efficiency, and/or other desired attributes orcharacteristics. The improved PAHs can be manufactured using novelmethods as disclosed herein and contain novel structures and components.

In the first step of an exemplary two-step photoelectrosynthetic andthermochemical process, molecular hydrogen is photoelectrosyntheticallyproduced by contacting a feed stream containing molecules havinghydrogen atoms with PAHs in the presence of sunlight. Light energy isused to drive the reduction half reaction in which hydrogen ions and/orwater molecules in the aqueous solution are reduced at the cathodes ofthe PAHs to form hydrogen gas. In a parallel half reaction, anions inthe solution are oxidized at the anodes of the PAHs to form oxidizedco-product(s). In a second step, hydrogen gas produced in the first stepis reacted with carbon dioxide in the presence of a thermal catalystunder appropriate reaction conditions to form a desired reductionproduct of carbon dioxide as the primary product (e.g., methane in anexothermic reaction with water as a co-product).

An exemplary photoelectrosynthetic system includes aphotoelectrosynthetic reactor vessel containing a hydrogen source andPAHs. The vessel includes a light permeable outer wall that allowssunlight to enter the vessel and interact with at least some of the PAHsin the vessel to produce an electric potential between the cathodes andanodes of the PAHs. The hydrogen source (e.g., waste stream containingcarbohydrates or acids, or electron donor reagents (SO₃ ²⁻ for example))provides protons that interact with PAHs energized by solar energy toproduce hydrogen at the cathodes of PAHs, which is collected in thevessel. Protons from the hydrogen source can be carried to the PAHcathodes by hydronium ions (H₃O⁺) in an acidic reaction environment orwater (H₂O) in a basic reaction environment. Molecules from the hydrogensource are oxidized at the anodes of PAHs to generate co-product(s). Theexemplary system further includes a reaction chamber or region in,attached to, or separate from the photoelectrosynthetic reaction vesselin which the hydrogen produced in the first reaction is reacted withcarbon dioxide in the presence of a catalyst as an exothermic process toform methane, which is collected for use or storage, or other reductionproducts of carbon dioxide so as to form, e.g., synthesis gas, methanol,formaldehyde or formic acid.

Exemplary PAHs that may be used in connection with the disclosed methodsand systems to photoelectrosynthetically produce hydrogen include one ormore layers of light absorbing p-type and/or n-type semiconductormaterial, an interface material between specific areas of thesemiconductor and an anode material, an anode material, an interfacematerial between other areas of the semiconductor and a cathodematerial, a cathode material, an interface material between other areasof the semiconductor and a protective coating, a protective coating onthe semiconductor material, and a hydrogen permeable coating on thecathode. The semiconductor material system(s) can consist of a p-njunction, a Schottky barrier to form a Schottky junction, and/or beattached to molecular absorbers.

The PAHs can be single- or multi-cell structures (e.g., to increaseelectrical potentials between the cathode and anode). They can bearranged on or otherwise attached to a substrate or suspended in areaction solution as individual particles (e.g., nanoparticles).

Exemplary methods for manufacturing PAHs include forming a semiconductormaterial as a particle or within a well or pore of a substrate, forminginterface layers on the semiconductor material, forming an anode on theinterface layer adjacent to the p-type semiconductor material, forming acathode on the interface layer adjacent to the n-type semiconductormaterial, forming a protective coating on exposed surfaces of thesemiconductor material, and forming a hydrogen permeable coating on thecathode. The foregoing steps are not necessarily performed in anyparticular order but rather according to the specific type of PAH beingmanufactured and the reaction sequences being employed. In the case ofSchottky junctions, the semiconductor material may be only one of ap-type or n-type material and wherein one of the metal electrode layerscan function as a Schottky barrier. Molecular absorbers may be usedalone or attached to a p-type or n-type semiconductor material and relyon ballistic charge transfer across the semiconductor material to createan electrical potential.

These and other embodiments and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only illustrated embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1A is a schematic perspective view of a prior artphotoelectrosynthetic “platelet” used to convert light energy intoelectrochemical energy;

FIG. 1B is a schematic cross-section view of a prior art Schottky-typephotoelectrosynthetic diode used to convert light energy intoelectrochemical energy;

FIG. 1C is a schematic cross-section view of a prior art p-n typephotoelectrosynthetic diode having a side-by-side configuration used toconvert light energy into electrochemical energy;

FIG. 1D is a schematic cross-section view of a prior art p-n typephotoelectrosynthetic diode having a stacked configuration used toconvert light energy into electrochemical energy;

FIG. 1E is a schematic cross-section view of a prior art apparatus usedto produce hydrogen gas and bromine liquid from a feed stream containingaqueous hydrobromic acid;

FIG. 2A is chart that schematically illustrates the spectral irradianceof sunlight as a function of wavelength;

FIG. 2B is chart that schematically illustrates the relationship betweenthe number of light photons in sunlight that are available forconversion into electrochemical energy as a function of semiconductorbandgap(s);

FIG. 3 is a flow diagram of an exemplary process for the production ofmethane from an organic waste stream and carbon dioxide;

FIG. 4 schematically depicts an exemplary reaction system for thephotoelectrosynthetic production of hydrogen from a feedstock thatincludes a hydrogen source and subsequent reaction of hydrogen withcarbon dioxide to form methane;

FIG. 5A is a schematic representation of an energy diagram showing how aband gap in a solar cell converts light energy into electrochemicalenergy to drive oxidation/reduction half reactions at the anode andcathode;

FIG. 5B is a schematic representation illustrating how placing p-typeand n-type semiconductor materials adjacent to each other in a solarcell causes a buildup of electrons (e⁻) in the n-type material and holes(h⁺) in the p-type material;

FIG. 5C is a schematic representation illustrating how incident lightcauses additional electrons to flow from the p-type material toward then-type material and thereby create an electric potential capable ofdriving an electrochemical reaction;

FIG. 6A is an energy diagram of semiconductor photoelectrochemistryinvolving an n-type semiconductor/liquid junction at equilibrium;

FIG. 6B is an energy level diagram for a p-n type PAH showing the flowof light-induced electrons and holes through the semiconductor materialsand electrodes;

FIG. 6C is an energy level diagram for a Schottky-type PAH showing theflow of light-induced electrons and holes through the semiconductormaterial and electrodes;

FIG. 7A is a cross-sectional schematic illustration of a PAH accordingto the disclosure for use in converting light energy intoelectrochemical energy in order to drive oxidation/reduction reactionsin a reaction mixture;

FIG. 7B is a cross-sectional schematic illustration of a multi-cell PAHaccording to the disclosure for use in converting light energy intoelectrochemical energy of higher voltage in order to driveoxidation/reduction reactions in a reaction mixture;

FIGS. 8A-8D schematically illustrate various exemplaryoxidation/reduction half reactions at the anode and cathode of a PAHaccording to the disclosure;

FIG. 9A schematically illustrates a process for manufacturing acylindrical PAH according to the invention that can drive half reactionsat the anode and cathode when exposed to light;

FIG. 9B schematically illustrates a process for manufacturing anano-sized particulate PAH according to the invention that can drivehalf reactions at the anodes and cathodes when exposed to light;

FIG. 9C schematically illustrates a process for manufacturing asheet-like PAH according to the invention that can drive half reactionsat the anodes and cathodes when exposed to light;

FIG. 10A illustrates a generic molecular absorber for converting lightenergy into electrochemical energy when exposed to light;

FIG. 10B illustrates another molecular absorber for converting lightenergy into electrochemical energy when exposed to light;

FIG. 11A schematically illustrates an exemplary PAH that includesmolecular absorber clusters attached to a semiconductor material forconverting light energy into electrochemical energy through ballisticcharge transfer;

FIG. 11B is a diagram that schematically illustrates the ballisticcharge transfer between the molecular absorber cluster and an electrodeon the semiconductor material;

FIGS. 12A and 12B show a set of plots that illustrates the success ofusing a protective coating to protect the semiconductor from theelectrolyte;

FIGS. 13A and 13B illustrate the use of a protective coating to avoidthe reduction of light transmission that comes from the formation ofwater condensation on the reactor surface;

FIG. 14 illustrates a checkerboard pattern of the reactors that can beused to mix the electrolyte;

FIG. 15A is a plot that illustrates the effect of using a Nafion coatingto prevent the unfavorable bromine back reaction;

FIG. 15B is a plot that illustrates the use of polyethylene glycol tocomplex bromine and reduce the rate of bromine back reaction; and

FIG. 16 illustrates an artificial photoelectrosynthetic device having ahoneycomb structure and housing isolated and autonomous light absorbingunits, which each have isolated anodes and isolated cathodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. Introduction

Photoelectrosynthetic processes are disclosed for producing hydrogen gasfrom organic waste streams or other sources of hydrogen involving theuse of photoelectrosynthetically active heterostructures (PAHs) andsunlight. Hydrogen produced via one or more photoelectrosyntheticprocesses is then reacted with carbon dioxide via a thermochemicalprocess to produce carbon-containing reduced products, including methaneand other alkanes, methanol, and carbon monoxide. The two step processesinvolving a photoelectrosynthetic reaction system and a thermochemicalprocess used to produce methane gas or other reduction products ofcarbon dioxide using the disclosed processes are also disclosed. Themethods and systems may employ PAHs known in the art or improved PAHshaving improved stability, energy conversion activity, and/or otherdesired attributes as disclosed herein.

The methods and apparatus disclosed herein solve needs relating toefficient solar energy conversion, which include systems which remainstable over time and are relative inexpensive compared to othertechnologies. Efficiency can be promoted by providing methods andsystems that are able to absorb a wider range of the solar spectrum,efficiently convert the absorbed solar irradiation energy into usefulfuels, and separate and store such fuels for later use. Capital costsare reduced by eliminating external circuitry as compared to anelectrolytic system that consists of photovoltaic cells and electrolyzercells used to produce hydrogen. Capital costs are also reduced by usinginexpensive and earth abundant materials in the disclosedphotoelectrosynthetic process. In addition, deactivation of a subset ofPAHs in a photoelectrosynthetic system does not affect the ability ofthe remaining PAHs to drive the desired photoelectrosynthetic reactions.

The solar energy conversion efficiency depends, in part, on thesemiconductor bandgap within the PAHs used to drive thephotoelectrosynthetic processes utilized herein. To illustrate thispoint, reference is made to FIGS. 2A and 2B. FIG. 2A depicts a solarradiation spectrum that compares spectral irradiance (W/m²/nm) as afunction of wavelength (nm). The jagged curve with the highest peaksdepicts the solar radiation spectrum at the top of the atmosphere. Thejagged curve with the lowest peaks depicts the solar radiation spectrumat sea level. The smooth, bold curve depicts a blackbody spectrum at5250° C. The spectral irradiance at sea level dips precipitously atwavelengths corresponding to absorption bands of water.

The ability of a PAH to absorb a particular wavelength depends on itsbandgap, which defines the potential of the electrons excited within thePAH. A semiconductor can only absorb photons with wavelengthscorresponding to energies that equal or exceed its bandgap energy. Thus,the smaller the bandgap, the greater is the range of photon wavelengthsthat can be absorbed by the PAH and the more photons are absorbed toexcite electrons. On the other hand, the bandgap is related to theelectrochemical potential (voltage) the PAH can produce, and with highervoltages available, thermodynamically there are a greater number ofelectrochemical reactions that can be performed. Thus, the smaller thebandgap, the smaller is the electrical potential of the electronsgenerated by the PAH and the fewer are the types of reactions that canbe powered by the PAH.

The importance of balancing the amount of light absorbed by thesemiconductor with semiconductor bandgap is illustrated in FIG. 2B. Thetotal power produced by a PAH can be expressed as Power=Np*Eg, where Npis the number of photons of sunlight absorbed by the PAH per second andEg is its bandgap energy. The large rectangular area in FIG. 2Brepresents a good balance between the rate of photons absorbed andenergy of created electron-hole pairs. The narrow rectangular area onthe top of the chart represents semiconductors that absorb more photonswith excessively low bandgap energy. The result is that the total usefulenergy generated by photons is low. The narrow rectangular area on thebottom of the chart represents semiconductors that have high bandgap butabsorb much less number of photons. Again, the result is low totaluseful energy. It can therefore be advantageous to select PAHs that areable to absorb a sufficient number of photons while having sufficientlyhigh bandgap energy to provide an optimal electrical energy level perphoton.

In addition to being able to drive useful redox reactions, the PAH mayitself undesirably react with substances in the reaction chamber. Thiscan cause corrosion and deactivation of the semiconductor device. Forexample, oxidation can convert silicon-containing semiconductormaterials to silicon dioxide. Conversely, semiconductor compounds likecadmium sulfide (CdS) can be reduced to cadmium metal (Cd). The tendencyto undergo photo corrosion is especially prevalent in the case of lowbandgap energy semiconductor devices, which limits their usefulness inmany reactions. It is therefore important to protect the semiconductordevices to promote longevity. The inability to avoid deactivation mayexplain why semiconductor devices known in the art have not been used incommercially viable process for producing useful fuels. Improvedsemiconductor devices that are better able to resist corrosion andremain active over longer periods of use are described more fully below.

II. Producing Desired Products from Hydrogen Source and CO₂

FIGS. 3 and 4 illustrate exemplary methods and systems for producingdesired products in a two-step process from a hydrogen source using PAHsto first absorb solar energy and produce hydrogen gas from the hydrogensource followed by the reduction of carbon dioxide using the hydrogengas to yield desired reduction products. Examples of desired reductionproducts include, but are not limited to, methane, methanol,formaldehyde, and synthesis gas, which can be used in Fischer-Tropschprocesses to make liquid fuels, diesel and/or paraffins.

FIG. 3 is a flow chart that depicts the basic steps of an embodiment foran exemplary process 300 for producing methane from an organic wastestream or other hydrogen source. A first step 302 involvesphotochemically oxidizing an organic waste stream or other hydrogensource to produce hydrogen gas and an oxidized co-product. For example,light energy is converted by PAHs in a reaction vessel into electricalenergy having sufficient voltage to drive a redox reaction involving thehydrogen source. Light energy is converted using semiconductor devicesknown in the art and/or photoelectrosynthetically activeheterostructures (PAHs) disclosed hereinafter, which can be dispersed asa suspension within the reaction vessel, packed as fixed bed, orattached to a floor or fixed surface in the reactor.

According to one embodiment, the half reaction in which hydrogencontaining ions or molecules are reduced to hydrogen gas are provided bywater molecules in either an acidic or basic environment. Production ofthe oxidized co-product in the other half reaction releases hydrogenions that go into solution and recombine with water molecules to formhydronium ions (H₃O⁺) in an acidic environment or water (H₂O) in a basicenvironment.

A second step 304 involves separating and collecting hydrogen gas fromthe reaction mixture. Because the reaction mixture will typically be aliquid or liquid-solid slurry and hydrogen has low solubility in aqueoussolutions, the hydrogen will form bubbles and rise to the surface of thereaction mixture or otherwise leave the liquid phase and collect as arelatively higher purity gas at the top of the reactor. From here, thehydrogen gas can either be utilized (reacted) within the reactor gasspace or be drawn off and utilized in a separate downstream reactor toform reduction products of carbon dioxide.

A third step 306 involves catalytically reacting hydrogen gas producedin step 2 and collected in step 304 with carbon dioxide to form desiredreduction products of carbon dioxide. Specific catalysts and conditionscan reduce the carbon dioxide with varying amounts of hydrogen toproduce several produces including but not limited to carbon monoxideand water, formic acid and water, methanol and water, or methane andwater. The last three reactions are exothermic and generally have highconversions of carbon dioxide, whereas the first (reverse water-gasshift reaction) is only minimally endothermic (+41 kJ/mole) and inexcess hydrogen will proceed to high carbon dioxide conversions atmodest temperature. Additional hydrogen can be added to the carbonmonoxide in order to product synthesis gas having a desired H₂/CO ratio.

The carbon dioxide can be provided in a wide range of concentrationsfrom various sources, discussed more fully below. Because hydrogenproduction using light energy is itself a relatively slow process, thekinetics of reacting hydrogen at modest temperatures produced by themethod with carbon dioxide can be similarly slow. This permits thereduction of carbon dioxide to be carried out at relatively lowtemperature and pressure, including within the head space of thereaction vessel used to photoelectrosynthetically produce hydrogen.Alternatively, hydrogen can be drawn off and sent to a downstreamreactor to produce methane or other reduction product(s) of carbondioxide. Moreover because hydrogen production relies on light energy,while the conversion of hydrogen and carbon dioxide into methane orother product(s) does not, the kinetics of the reaction used to producemethane or other product(s) can actually be slower than the rate atwhich hydrogen is produced. For example, hydrogen that builds up morequickly during daylight hours can be used up during night time toproduce desired reduction product(s) of carbon dioxide. Water producedas a co-product can be condensed within the vessel where methane isproduced and returned to the solution, or else it can be channeled backinto the reaction vessel from a downstream reactor used to producemethane or other reduction product(s).

A fourth step 308 involves collecting the methane or other product(s)product for further processing or use. The methane gas or otherproduct(s) will typically be removed from the reactor as it is producedin order to further drive the reaction forward according to the LeChatelier principle. According to one embodiment, the head space in thephotoreactor can be completely purged with carbon dioxide at dawn aftermethane or other product(s) are formed during the night and just beforeproduction of hydrogen resumes with the rising of the sun. The methaneor other product(s) produced in step 308 may contain impurities andtherefore require cleaning processes to remove such impurities.

An optional fifth step 310 involves reacting the reduction product(s) ofcarbon dioxide and hydrogen to yield additional product(s). For example,synthesis gas can be utilized in one or more Fischer-Tropsch processesto form one or more liquid hydrocarbon fuels, including, but not limitedto, diesel, naphtha, or other paraffins.

FIG. 4 illustrates an exemplary system 400 for producing methane from ahydrogen source, sunlight and a carbon dioxide stream. A feedstock 402provides a source of hydrogen that can be photoelectrosyntheticallyconverted into hydrogen gas using PAHs or other semiconductor lightabsorbers capable of converting light energy into electrical energy. Thefeedstock can be one or more of a number of different waste streams,including, but not limited to, waste streams from complex organicchemical industries (e.g., pharmaceutical processing, pesticidemanufacturing, hydrocarbon refining, detergents, plastics, pulp andpaper mills, textile dyes), agricultural, biofuels, chemicalmanufacturing (e.g., toxic hydrogen sulfide, hydrogen bromide, hydrogenchloride), municipal wastewater, iron and steel industry, coal plants,and tannery.

In general, the feedstock 402 includes water with some sort ofsupporting electrolyte to provide conductivity. The waste stream mayinclude organic or inorganic molecules such as, but not limited to, oneor more of celluloses, hydrocarbons, non-biocompatible pollutants,alcohols, ethanol, methanol, isopropyl alcohol, pesticides, glucose,phenols, carboxylic acids, mineral acids, cyanide, ammonia, acetic acid,dyes, surfactants, chlorophenols, anilines, oxalic acid, and tartaricacid.

The feedstock 402 is introduced into a reaction vessel 404 together withPAHs or other semiconductor light absorbers capable of converting lightenergy into electrical energy. The reaction vessel 404 will generallyhave transparent walls that permit light radiation 406 from the sun topenetrate into the reaction mixture, where it can be converted intoelectrical energy by the PAHs or other semiconductor light absorbers inorder to drive oxidation and reduction reactions.

According to one embodiment, the reaction vessel 404 may comprise aflexible polymer wall or chamber that is placed over a relatively largesurface area and that has a low height compared to the length and widthof the vessel 404 in order to maximize the amount of incident light thatcomes into contact with the reaction mixture and PAHs. A conduitintroduces the feedstock 402 at one end, which is allowed or caused toslowly flow through the vessel 404 and then be removed from the otherside. The rate at which the feedstock passes 402 through the vessel canbe a function of the rate at which the hydrogen containing molecules areconverted into hydrogen and oxidized co-product(s) 406. Thus, thefeedstock may pass through the reaction vessel 404 during the day butnot during the night when there is no sunlight to drive the reaction.Moreover, the kinetics of hydrogen production may itself changedepending on such things as the angle and/or intensity of incidentsunlight, the amount of cloud cover, the temperature of the reactionmixture, the opacity or transparency of the reaction mixture, and thelike.

The reduced hydrogen product can be collected from or within thereaction vessel 404. In one embodiment, the hydrogen gas rises to thetop of the reaction mixture and collects in the head space of thereaction vessel 404. The oxidized co-product(s) 406 can be drawn offfrom the reaction vessel 404 together with the spent feedstock or, ifpossible, separated from the feedstock and drawn off from the reactorbefore the spent feedstock is removed. Where the oxidized co-product isvaluable, it may be desirable to separate it from the spent feedstock.In other cases, it is simply part of the spent feedstock and remains forfurther downstream processing.

The nature of the oxidized co-product 406 may depend largely on the typeof feedstock 402 that is processed. Examples of oxidized co-products 406include, but are not limited to, biodegradable products produced fromnon biodegradable organic waste streams, biocompatible organics whichcan be biologically treated in a downstream process, oxalic acid,halogens, bromine, sulfur and sulfur containing ions, nitrogencontaining ions, metal containing ions, chlorine and detoxified water.By way of example, photoelectrosynthetically processing ethanol using aPAH and sunlight forms hydrogen as the main product and acetaldehyde asthe oxidized co-product 406. Processing hydrogen sulfide can yieldelemental sulfur as the oxidized co-product 406. Hyrobromic acid yieldsbromine as the oxidized co-product 406. Decomposition of oxalic acidyields carbon dioxide as the oxidized co-product 406. Processing oftartaric acid can yield tartronaldehydic acid and carbon dioxide asco-products and/or glyoxal and carbon dioxide. Glyoxal can, in turn, beoxidized to oxalic acid and carbon dioxide.

The hydrogen from reaction vessel 404 is mixed with a carbon dioxidecontaining stream 408 in reaction vessel 410 in the presence of acatalyst in order to thermochemically reduce the carbon dioxide to formone or more reduction products of carbon dioxide 412 and a waterco-product 414. The carbon dioxide stream 408 can come from a variety ofdifferent sources, including, but not limited to, flue gases fromboilers, power plants, cement kilns, or industrial processes, naturalgas containing excessive quantities of carbon dioxide, biogas containinga mixture of methane and carbon dioxide, and other gaseous streams.

The reaction between hydrogen and carbon dioxide can occur within thesame reaction vessel 404 used to photoelectrosynthetically producehydrogen. It may be performed in the headspace of vessel 404, forexample, or a separate chamber of vessel 404. Alternatively, thehydrogen may be drawn off from vessel 404 and introduced into a separatereaction vessel together with the carbon dioxide containing stream 408.In the latter case, it may be possible or desirable to increase thepressure and/or temperature of the reactants in order to speed up thereaction kinetics.

The catalyst used to catalyze the reaction between hydrogen and carbondioxide to form methane can be ruthenium, nickel or other usefulcatalyst known in the art for this type of reaction. Examples include,but are not limited to, noble metal or alloy supported catalysts (noblemetal)/(solid acidic support), such as Pt/SiO₂, Pt/γ-Al₂O₃, Pd/SiO₂,Pd/γ-Al₂O₃, Pt/Zeolite, or (Pt-Pd)/γ-Al₂O₃, nickel on silica-alumina,ruthenium (less than 5 nm will react at room temperature). Additionalexamples are metal oxide catalysts, including platinum group dopedcerium oxide and magnesium oxide and Ru doped cerium oxide. The type ofcatalyst may be selected according to desired kinetics and selectivityaccording to the reaction conditions during methane formation.

An exemplary catalyst used for the reaction between hydrogen and carbondioxide to produce synthesis gas from carbon dioxide and hydrogen, inone example, can be a silica based catalysts consisting of 5% (weight)copper and 0.5% nickel. The only products are CO with 60% conversion ofCO₂ to CO at 350° C., 150 torr, and a CO₂/H₂ feed ratio of ¼. Higherselectivity and conversions are achieved with greater hydrogenconcentrations. The source of hydrogen for the synthesis gas can simplybe additional or make up hydrogen as photoelectrosynthetically producedherein.

The synthesis gas may then be reacted on metal catalysts including iron,cobalt, ruthenium, iridium, osmium, and their combinations to producelinear alkanes with 2 or more carbons and water by way of theFischer-Tropsch pathway.

An exemplary catalyst used for the reaction between hydrogen and carbondioxide to produce methanol from carbon dioxide and hydrogen typicallycontains copper and often other metal oxides on alumina or silicasupports. In one example, Cu/ZnO/Al₂O₃ is used in a reaction operated atbetween 200 and 400° C. and pressures of 10-1000 psi.

The water co-product 314 produced during thermochemical reduction ofcarbon dioxide for the formation of methane can be collected as purewater or, alternatively, be recycled back into vessel 404 as part of thereaction mixture. In the case where hydrogen is collected and used toproduce methane in the head space of vessel 404, the water can simplycondense and fall back into the reaction mixture by the force ofgravity. Alternatively, if methane is produced in a separate vessel,some or all of the water co-product 314 can be returned to the reactionvessel 404 using appropriate conduits or channels known in the art.

III. Photoelectrosynthetically Active Heterostructures

Photoelectrosynthetically Active Heterostructures (PAHs) that areespecially useful in the foregoing processes and systems forphotoelectrosynthetically producing hydrogen from a hydrogen source andsolar energy are those which are photo-electrochemically efficient anddurable such that they can withstand the aqueous chemical environment ofthe reactor for extended periods without significant deactivation. Inaddition, it is desirable that they can be manufactured at a reasonablecost from relatively inexpensive elements commonly used to formsemiconductors. Where more expensive metals, such as platinum group andrare earth metals are used, the relative quantity of such materialscompared to the quantity of semiconductor materials can be substantiallylower.

By way of background, FIGS. 5A-5C schematically illustrate theunderlying manner in which semiconductor materials are able to convertphotonic energy into an electrical potential in order to drive anelectrochemical reaction. FIG. 5A shows a simple bandgap diagram 500 ofa semiconductor material having a conduction band 502 and a valence band504. The difference between them is the band gap energy 506. A cathode508 a in electrical contact with the conduction band 502 receiveselectrons from the semiconductor that can be used to drive reductionhalf reaction 512 a (e.g., hydrogen ions to hydrogen gas). An anode 508b in electrical contact with the valence band 504 receives holes fromand returns electrons to the semiconductor and in so doing drivesoxidation half reaction 510 b (e.g., anions to oxidized co-product). Thevoltage between the cathode 508 a and anode 508 b is slightly less thanthe bandgap potential 506 of the semiconductor.

FIGS. 5B and 5C are references from Engineering 100, “Disorder andCoherence: From Light Bulbs to Lasers,” Sep. 14, 2008, Matt Greco,Nirala Singh and Kevin Wentzke (under the supervision of Dr. JohnHinckley and Prof. Jasprit Singh). FIG. 5B schematically illustrates asemiconductor 520 of a solar cell having a p-n junction comprised of ap-type semiconductor material 522 and an n-type semiconductor material524. The n-type semiconductor has excess freely-moving negativeelectrons530, and the p-type has excess positive charges, or “holes” 528 (whichare really the absence of electrons, the actual carriers of positivecharge, protons, do not move). When these two materials come intocontact, there is an electric potential barrier 526 that forms andseparates the electrons and holes, keeping them from combining. Althoughboth sides 522, 524 have charges that move freely, the sides areelectrically neutral.

FIG. 5C shows what happens when the semiconductor 520 in a solar cell isexposed to radiation energy 532. When sunlight 532 falls on thesemiconductor 520, additional electrons are freed on both sides of thejunction, but the potential barrier 526 only allows electrons 534 to goin one direction, from the p-type semiconductor material 522 toward then-type semiconductor material 524, as shown by the arrows. This transfercauses a build-up of voltage across the cell, and if a circuit(including electrochemical equivalent circuits) is attached to thesemiconductor 520, current will flow. The semiconductor 520 acts as avoltage source and a source of current.

FIG. 6A-6C are energy diagrams of semiconductors when used in connectionwith PAHs according to the disclosure. FIG. 6A is an energy diagram ofan n-type semiconductor/liquid junction at equilibrium. In frame (a),before charge equilibrium occurs, the energy levels of the semiconductorconduction and valence bands are uniform at all points along the axis.In frame (b), after charge equilibrium has occurred, a depletion layeris formed in the semiconductor. As shown, the electric potential energylevels of E_(cb) and E_(vb) are dependent on the distance from thesemiconductor solution interface. However, at equilibrium, theelectrochemical potential is the same in the solution and at all pointsin the semiconductor (i.e., E(A/A⁻)=E_(F)), where E_(F) is the Fermilevel. The parameter qVn is defined as the difference between E_(F) andE_(cb) in the bulk semiconductor, and V_(bl) is the built-in voltage ofthe junction. The parameter φ_(b), the barrier height, is defined asφ_(b)=V_(n)=V_(bl).

To equilibrate Fermi levels, the semiconductor donates electrons to theelectrolyte, giving itself a net positive charge near the surface of thesemiconductor, and a net negative charge at the surface of theelectrolyte. This causes the valence and conduction bands in the bulk ofthe semiconductor to lower in energy. However, the field in the regionwhere there is a net positive charge causes the energy of the conductionband and valence band to remain higher, causing a band bending effect inwhat is called the depletion region. The E_(cb) and the E_(vb) at thesemiconductor surface remains constant relative to the Fermi level ofthe solution.

FIG. 6B illustrates an energy level diagram 600 for a p-n typesemiconductor 602 in a reaction medium. The semiconductor 602 iscomprised of a p-type semiconductor 602 a and an n-type semiconductor602 b, which can be separated by one or more ohmic contacts 604. Thebandgap 606 is related to the Fermi leval 607. Light having energy (hν)is absorbed in both halves of the semiconductor 602, creatingelectron-hole pairs in the p-type semiconductor 602 a and in the n-typesemiconductor 602 b. Electrons 608 a having a potential related to thebandgap 606 are provided at cathode 610 a in order to drive reductionhalf reaction 612 a (A⁺+e⁻). Holes 614 a move away from the cathode 610a through the semiconductor materials toward the n-type semiconductor602 b. At the anode 610 b, holes 614 b drive oxidation half reaction 612b (B⁻+h⁺), which gives up electrons 608 b, which maintain the chargebalance within the semiconductor 602.

FIG. 6C illustrates an energy level diagram 620 for a Schottky-typesemiconductor in a reaction medium. In this example, the semiconductoris comprised of an n-type semiconductor 622 and a metal Schottky barrier623, which can be separated by one or more ohmic contacts 624. Thebandgap 626 is related to the Fermi leval 627. Light 625 having energy(hν) greater than the bandgap 626 is absorbed by semiconductor 622,creating electron-hole pairs in the n-type semiconductor 622. Electrons628 a having a potential related to the bandgap 626 are provided atcathode 630 a in order to drive the reduction half reaction 632 a(A⁺+e⁻). Holes 634 move from the n-type semiconductor 622 toward theanode 630 b to drive the oxidation half reaction 632 b (B⁻+h⁺), whichgives up electrons 628 b, and which maintain the charge balance withinsemiconductor 622. In the case where the semiconductor is comprised of ap-type semiconductor, the flow of electrons and holes is reversed.

FIGS. 7A and 7B illustrate exemplary PAHs according to the disclosure.FIG. 7A is a schematic cross-sectional representation of a PAH 700 thatincludes a semiconductor absorber 702 for absorbing light energy. Thesemiconductor absorber 702 may comprise one or more types ofsemiconductor materials (e.g., p-type and/or n-type) to form one or morep-n junctions or one or more Schottky junctions.

Examples of suitable p-type semiconductor materials include, but are notlimited to intrinsic or p-doped SnS, ZnS, CdS, CdSe, CdTe, Cu₂S, WS₂,Cu_(x)O, Cu₂ZnSnS₄, CuIn_(x)Ga_(1-x)Se₂, GaN, InP, SiC, and othersselected from the classes of doped (p-type) or undoped i) elementalsemiconductors including Si, and Ge, and ii) compound semiconductorsincluding, metal sulfides, selenides, arsenides, nitrides, antinomides,phosphides, oxides, tellurides, and their mixtures containingrespectively, sulfur (S) selenium (Se), arsenic (As), antimony (Sb),nitrogen (N), oxygen (O) tellurium (Te), and/or phosphorus (P) as one ormore electronegative element(s) A, and one or more metals, M, of theform M_(n)A_(x) where M is one or a combination of elements includingbut not limited to Cu, Ga, Ge, Si, Zn, Sn, W, In, Ni, Fe, Mo, Bi, Sb,Mg.

Examples of suitable n-type semiconductor materials include, but are notlimited to intrinsic or n-doped InS, CdTe, CdS, CdSe, CdTe, Cu₂S, WS₂,Cu_(x)O, Cu₂ZnSnS₄, CuIn_(x)Ga_(1-x)Se₂, GaN, InP, SiC, and othersselected from the classes of doped (n-type) or undoped i) elementalsemiconductors including Si, and Ge, and ii) compound semiconductorsincluding, metal sulfides, selenides, arsenides, nitrides, antinomides,phosphides, oxides, tellurides, and their mixtures containingrespectively, sulfur (S), selenium (Se), arsenic (As), antimony (Sb),nitrogen (N), oxygen (O), tellurium (Te), and/or phosphorus (P) as oneor more electronegative element(s) (A), and one or more metals (M), ofthe form M_(n)A_(x) where M is one or a combination of elementsincluding but not limited to Cu, Ga, Ge, Si, Zn, Sn, W, In, Ni, Fe, Mo,Bi, Sb, Mg.

A protective coating 704 covers semiconductor absorber 702 and contactsthe semiconductor at interface 706 and protects absorber 702 fromcorrosion or degradation. The protective coating is advantageouslytransparent to light energy that is to be absorbed by semiconductorabsorber 702 and may comprises any suitable electrically insulatingmaterial, including, but not limited to Al₂O₃, SiO₂, ZrO, AlF₃, andTiF₂, ZnO, and TiO₂.

The interface 706 where the semiconductor contacts the protectivecoating is specifically designed to minimize electron/hole recombinationand may be formed by either pretreatment of the semiconductor orprotective coating material to eliminate charge recombination sites. Aspecific example is the application of a thin layer of ZnO by atomiclayer deposition from a chemical vapor precursor to serve as theinterface between an alumina protective coating and the semiconductorsurface to minimize surface electronic trap states.

A cathode 708 is positioned adjacent to semiconductor absorber 702 atinterface 710. The cathode serves as an electrocatalyst to efficientlytransfer electrons from the cathode bulk to electron acceptor reactantsin solutions. Suitable materials for use in forming cathode 708 include,but are not limited to, conductors comprised of platinum group metalssuch as Pt and Au, transition metals, transition metal oxides (e.g.NiO), metal carbides (e.g., WC), metal sulfides (e.g. MoS₂), andelectrical conducting carbon containing materials, such as graphite,graphene, and carbon nanotubes.

A selectively permeable barrier 716 covers the cathode 708, which allowsonly molecular hydrogen to pass and, in systems used with acidic media,hydrogen ions (or hydronium ions) and molecular hydrogen and, in systemsused in basic media, water and hydroxyl ions. The barrier prevents thereduction of other species (e.g. products from the anode) in thereaction mixture on the cathode which would decrease the efficiency ofthe PAH. The selectively permeable barrier layer 716 is an electricalinsulator and may be comprised of hydrogen permeable barrier materialswhich are also permeable to the other species used for otherapplications and known by those skilled in the art, examples of whichinclude chromium (III) oxide (Cr₂O₃), Nafion membranes (made fromsulfonated tetrafluoroethylene-based fluoropolymer-copolymers), andhydrogen permeable polymers such as acrylics, also an element of theinvention are new metal oxide hydrogen ion/molecule conductors comprisedof mixtures of metal oxides, including WCrO_(x,) and WZrCeO_(x).

The interface 710 between the cathode 708 and the semiconductor 702provides an electrically conductive pathway between the semiconductorand the electroreduction catalysts cathode. To maintain a highefficiency this interface must minimize the resistance and chargerecombination. The specific material will depend upon the type ofsemiconductor and the cathode material. For example, if thesemiconductor were n-type silicon and the cathode were platinum theinterface could be hydrogen terminated silicon, Si—H, prepared bytreating the Si in dilute buffered HF solution with a layer of Ti toserve as an ohmic contact or (n-Si/H/Ti/Pt).

An anode 712 is positioned on the opposite side of semiconductorabsorber 702 at interface 714. The anode serves as an electro-oxidationcatalyst to facilitate the chemistry and electron transfer from theelectron donating reactants which are oxidized to the cathode bulk. Thematerials which may be used to form the anode 712 include, but are notlimited to conductors comprised of metals, their oxides, and theirmixtures from metals including Ru, Ag, V, W, Fe, Ni, Pt, Pd, Ir, Cr, Mn,Cu, Ti, and metal sulfides (e.g., MoS₂) and electrical conducting carboncontaining materials such as graphite, graphene, and carbon nanotubes.

The interface 714 between the anode 712 and the semiconductor 702provides an electrically conductive pathway between the semiconductorand the electro-oxidation catalyst anode. To maintain a high efficiencythis interface must minimize the resistance and charge recombination.The specific material will depend upon the type of semiconductor and theanode material. For example, if the semiconductor were n-type siliconand the anode were a platinum Schottky barrier, the interface can behydrogen terminated silicon (Si—H), prepared by treating the Si indilute buffered HF solution for a junction of the form n-Si/H/Pt.Alternatively, 1-2 atomic layers of aluminum or Mg may be applied at theinterface of p-type copper oxide and a ruthenium oxide anode.

As further illustrated, the reduction half reaction of hydrogen ions toform hydrogen gas occurs at the cathode 708, and the oxidation halfreaction to form the oxidized co-product occurs at the anode 712. Whilethe oxidation half reaction is illustrated as anion (A⁻) being oxidizedto product (A), it could be understood that any molecule of any valencecan be reduced to any oxidized product of reduced valence and/or lowerhydrogen content.

FIG. 7B shows an alternative embodiment of a PAH 720 having multiplelight absorbers 722 a, 722 b, 722 c, 722 d are connected in series bysemiconductor contacts 723. Each light absorber 722 produces it ownvoltage potential (e.g., 0.5V as shown), and when connected in series,the total voltage will be the sum of individual voltages (e.g., 2.0V asshown). The light absorbers 722 can be p-n junctions and/or Schottkyjunctions.

A protective covering 724 is attached to light absorbers 722 atinterface 726 and protects light absorbers 722 from corrosion or otherdeleterious reactions during use. A cathode 728 is attached at one endof the light absorber series 722 by interface 730 an anode 732 isattached to the opposite end of the light absorber series 722 byinterface 734. A hydrogen-permeable coating 736 covers cathode 730.

FIGS. 8A-8D illustrate exemplary oxidation/reduction reactions that canoccur at the cathode and anode of a PAH according to the disclosure. Inthe reaction shown in FIG. 8A, ethanol (C₂H₆O) isphotoelectrosynthetically oxidized to acetaldehyde (C₂H₄O) at the anodeas co-product, and hydrogen gas (H₂) is produced at the cathode as themain product. Because the reaction environment is alkaline in thisexample, water (H₂O) is reduced to hydrogen gas (H₂) and hydroxyl ions(OH⁻), and hydrogen ions (H⁺) are removed from ethanol when oxidized toacetaldehyde and combine with hydroxyl ions (OH⁻) to maintain chargebalance in the reaction mixture.

In the reaction shown in FIG. 8B, hydrogen sulfide (H₂S) isphotoelectrosynthetically oxidized to elemental sulfur at the anode asco-product, and hydrogen gas is produced at the cathode as the mainproduct. Because the reaction mixture is acidic in this example,hydronium ions (H₃O⁺) are reduced to hydrogen gas (H₂) and water (H₂O),and hydrogen ions (H⁺) are removed from hydrogen sulfide when oxidizedto elemental sulfur and combine with water to maintain charge balanceand acidity in the reaction mixture.

In the reaction shown in FIG. 8C, hydrobromic acid (HBr) isphotoelectrosynthetically oxidized to elemental bromine (Br₂) at theanode as co-product, and hydrogen gas is produced at the cathode as themain product. Because the reaction mixture is acidic in this example,hydronium ions (H₃O⁺) are reduced to hydrogen gas (H₂) and water (H₂O),and hydrogen ions (H⁺) are removed from hydrobromic acid when oxidizedto elemental bromine and combine with water to maintain charge balanceand acidity in the reaction mixture.

In the reaction shown in FIG. 8D, oxalic acid (H₂C₂O₄) isphotoelectrosynthetically oxidized to carbon dioxide (CO₂) at the anodeas co-product, and hydrogen gas is produced at the cathode as the mainproduct. Because the reaction mixture is basic in this example, water(H₂O) is reduced to hydrogen gas (H₂) and hydroxyl ions (OH⁻), andhydrogen ions (H⁺) are removed from oxalic acid when oxidized to carbondioxide and combine with hydroxyl ions to maintain charge balance in thereaction mixture.

FIGS. 9A-9C illustrate how exemplary PAHs according to the disclosurecan be manufactured and used to drive desired oxidation/reductionreactions. FIG. 9A illustrates in a schematic process diagram 900 in theformation of PAHs which may be substantially cylindrical similar tothose shown in FIGS. 7A-8D described above. Initially, a structure 902formed of inert insulating material (e.g., alumina) is provided, whichincludes a hollow interior cavity. The cavity is filled with one or moretypes of semiconductor materials 904 (e.g., p-type and/or n-type) inorder to form one or more p-n junctions and/or one or more Schottkyjunctions within the hollow cavity of tubular structure 902. This may beperformed by any method known in the art for forming semiconductorjunctions, including, but not limited to electrodeposition, chemicaldeposition and physical vapor deposition. In one specific example, thecylinder is silica and is provided atop an electrically conductingsurface consisting of copper. The semiconductor material 904 comprisedof CdS is electrodeposited into the cylinder. Then the copper etchedaway leaving free floating CdS filled cylinders.

A cathode layer 906 is formed on an exposed side of semiconductormaterial(s) 904 (e.g., by one or more of photodeposition,electrodeposition, annealing or heat treating steps), and an anode layer908 is formed on the other exposed side of semiconductor material(s) 904(e.g., by electrodeposition or photodeposition followed by oxidation). Ahydrogen permeable membrane 910 is applied over the cathode layer 906 inorder to prevent back reaction and/or formation of undesirable reductionproducts.

A specific example would be to place the CdS filled cylinders in anelectrolyte solution with H₂PtCl₆ and a sacrificial oxidant (methanol)and exposing the solution to light. Platinum metal would be photoreducedon the semiconductor surface to form the cathode 906. The electrolytewould then be changed to chromium sulfate/chloride and methanol and thesolution exposed to light. Chromium would be photoelectroreduced ontothe Pt, forming a thin layer of metal. Then the solution would bechanged to nickel acetate and a sacrificial electron acceptor and thesolution exposed to light. NiOOH is anodically deposited from lightgenerated holes to produce the precursor to layer 908. The PAH is thenheat treated to convert the chromium on the Pt layer to chromium oxideto form layer 910 and the NiOOH to NiO to form layer 908.

FIG. 9B illustrates an exemplary flow diagram 920 for use in formingnano-sized particulate PAHs. First, a semiconductor particle 924 isformed or provided and reacted with a cathode metal source to formcathode particles 926 on the surface of the semiconductor particle 924.In one example, chloroplatinic acid (H₂PtCl₆) is photochemically reducedin a reaction medium that includes methanol as the sacrificial oxidizedspecies to form the cathode metal clusters 926 on p-type siliconparticles which were initially hydrogen terminated by treatment with HF.The spacing of the cathode metal clusters 926 is controlled by formingthe initial or seed particles, by limiting the amount of light used anduse of low concentrations of chloroplatinic acid to initially form theplatinum seed particles. Thereafter, the reduced platinum metal atomswill preferentially deposit over the seed particles and grow the cathodemetal clusters 926.

A hydrogen permeable coating 930 is then selectively applied over thecathode metal clusters 926. This may be performed, for example, in amulti-step process that includes a first step of photochemicallyreducing copper metal onto the surface of the cathode metal clusters926, followed by electroless exchange of copper metal atoms withchromium metal ions to form chromium metal, followed by oxidation of thechromium metal to form chromium oxide. Alternatively, the hydrogenpermeable coating 930 can be formed over the cathode metal clusters 926in a subsequent step after formation of the anode metal clusters 928. Aspecific example is to deposit onto the reduced Pt cathode metalcatalysts chromium metal by photoelectrodeposition from a chromiumsulfate/chloride solution.

To form the anode metal clusters 928, an anode metal source is providedand reduced onto the surface of the semiconductor particle 924. In oneexample, ruthenium chloride (RuCl₃) is partially hydrolyzed in an acidmedium in the presence of light to form ruthenium hydroxide, which islater dehydrated to form the ruthenium oxide anode. In another example,ruthenium acetate is used as the source of anodic deposition ofruthenium oxyhydroxide by exposure of the particles in ruthenium acetatesolution to light.

The semiconductor particle 924 with attached cathode particles 926 andanode particles 928 is oxidized (e.g., using an oxidizer and heat in anaqueous medium). For example, if the semiconductor particle 924 includessilicon, oxidation forms a passivated layer of silicon dioxide on allpreviously unprotected semiconductor surface not covered by cathodeparticles 926 and anode particles 928 in order to form a protectivelayer 922 that protects the semiconductor material 924 during use. Inthe example above, the oxidation step finishes forming the chromiumoxide on the cathode and the ruthenium oxide on the anode.

In use, when the PAH is exposed to light energy that equals or exceedsthe bandgap energy of the PAH, the cathode particles 926 drive thereduction half reaction in which hydrogen ions are reduced to hydrogengas. The anode particles 928 drive the oxidation half reaction usingmolecules in the waste stream material to form an oxidized co-product.

FIG. 9C illustrates an exemplary process diagram 940 for use in formingsheet-like PAHs. First, a layered conducting layer 944 is provided, onwhich an insulating honeycomb structure 946 is deposited to formcavities into which materials can be deposited and processed,alternatively the honeycomb structure 946 can be formed from a singleconducting substrate such as aluminum which can be anodized to forminsulating partitions 946 and open pores using methods know to the artand the assembly laminated or formed on a conductor. As shown in FIG.9C, the altered layered sheet comprises a non-oxidized conducting layer944 (e.g., gold) and an insulating layer 946 having spaced-apartcavities.

In a first step, a protective layer 948 (AlF₃) may be formed onto thesurface of insulating structures 946. In a second step, the remainingrecesses are filled with one or more types of conductors(anodes/cathode, interfaces, semiconductor material (e.g., one or moreof p-type or n-type)) to form one or more absorber substructure cells orregions 950. Thereafter, a second electrode material 952 is depositedonto the exposed surface of the semiconductor material. Whether thesecond electrode material 952 functions as a cathode or anode depends onthe type of semiconductor material (p-type or n-type) that is adjacentto the second electrode material 952. The interfaces between theanode-semiconductor and cathode semiconductor and semiconductorprotective coating will depend on the types of semiconductor.

In a third step, a hydrogen permeable material 954 is applied to thesurface of the sheet where the cathode is located. In the case where thenon-oxidized metal layer 944 serves as the cathode, a hydrogen permeablelayer 954 a is applied over the exposed surface of the non-oxidizedmetal layer 944. Alternatively, in the case where the second electrodematerial 952 serves as the cathode, a hydrogen permeable layer 954 b isapplied over the exposed surface of the second electrode material 952.

Thereafter, at least a portion of the oxidized metal layer 946 can beetched to form a porous structure 956 having greater surface area andtransparency for absorbing incident light. In use, when the PAH isexposed to light energy that equals or exceeds the bandgap energy of thePAH, the cathode layer or regions 958 drive the reduction half reactionin which hydrogen ions are reduced to hydrogen gas. The anode layer orregions 960 drive the oxidation half reaction using molecules in thewaste stream material to form an oxidized co-product.

FIGS. 10A and 10B illustrate alternative examples of molecular lightabsorbers. Molecular light absorbers are designed have the same functionas chlorophyll in plants in order to drive desired oxidation/reductionreactions when exposed to light energy. They play the same role as thesemiconductor in the previously described PAH. One form of PAH describedherein uses the molecular light absorber to convert sunlight intoelectrons and holes and either transfer the electrons and holes throughinterfaces connected to anodes and cathodes similar to those describedabove or to molecular level catalytic centers within the absorber.

FIG. 10A depicts a generic molecular light absorber 1000 having a metalcenter (M) attached to ligands (e.g., 4), which are able to createdelocalization of electrons and buildup of charge potential when exposedto light energy. As shown, incident light of energy, hν, causes anelectron to be discharged in the vicinity of ligand L₁ and a hole to bedischarged in the vicinity of ligand L2.

FIG. 10B depicts a molecular light absorber species 1002 based onmanganese oxalate coordinated or complexed with ligands L₁, L₂, phenyl,and ammonia, which are able to create delocalization of electrons andbuildup of charge potential when exposed to light energy. The ammoniaand phenyl may be replaced by inorganics providing similar electronicmodifications. As shown, incident light energy of energy, hν, causes anelectron to be discharged in the vicinity of ligand L₁ in order to drivethe reduction of hydrogen ions to hydrogen. A hole is discharged in thevicinity of ligand L₂ in order to drive the oxidation of anion A⁻ (orother reactant) to A or other reduced oxidation co-product species.

FIGS. 11A and 11B depict an exemplary PAH system 1100 that includemolecular absorbers 1108 attached to a wide gap 1104 semiconductorparticle 1102 by interface layers 1106. The semiconductor particle 1102is used for charge separation and not absorption and is comprised of awide bandgap 1104 materials (WO₃, ZnO, Ti0 ₂). In this example, themolecular absorbers 1108 absorb light and generate electrons or holesthat are transferred to the semiconductor particle 1102. N-type wide gapsemiconductors are used as electron acceptors and p-type wide gapsemiconductors for hole acceptors. The hole or electron remaining withinthe molecular absorber is transferred to the reactants in solution byway of transfer moieties which are part of the molecular absorber 1108.The electrons discharged at cathode sites 1110 drive the reduction halfreaction of hydrogen ions to hydrogen. Holes migrate to anode sites,which drive the oxidation half reaction. It is the molecular absorber,not the semiconductor particle 1102, which absorbs light energy andproduces the electrical voltage potential across the semiconductorparticle 1102. A protective layer 1112 protects the semiconductorparticle 1102 during use.

In addition to the general descriptions of the embodiments of thepresent invention mentioned above, a list of examples is provided toillustrate in detail some of the embodiments of the present invention.It should be respected by those proficient in the art that the abovediscussion and the techniques disclosed in the following systems, alongwith processes which follow the techniques discovered by the inventors,should be considered as exemplary prototypes and can be made by makingmany or slight changes in the disclosed embodiments to obtain alike orparallel results without deviating much from the essence and scope ofthe invention. The first three examples discuss methods of forming aphotoelectrosynthetically active heterostructures. The fourth examplerelates to one of the possible reactor systems that can be used for asolar energy conversion system. The fifth example relates to methods forreducing the rate of backreaction using protective coatings orcomplexing agents.

EXAMPLE 1 An Exemplary Stable Artificial Photoelectrosynthetic (PS)Device for Production of Fuels and Chemicals

The basic element of an artificial photosynthetic system is anindependent photoelectrosynthetically active heterostructure (PAH)consisting of semiconductor absorber material protected by a transparentfunctional coating material with exposed cathode and anodeelectrocatalyst contacts. In general, processes for fabricating a stableartificial photoelectrosynthetic (PS) system comprise the steps: i)developing a high efficiency semiconductor light absorbers with bandgaps matched to maximize solar spectrum absorbance, ii) developing aninorganic material stabilizing the semiconductor in the electrolytemedia, thus allowing the use of less expensive and earth abundantsemiconductor materials, iii) developing specific electrocatalysts formaximum efficiency formation of electrocatalytic products, and iv)developing a semi-selective electrocatalyst coating to allow use ofhomogeneous slurry reactors.

In one embodiment, the artificial photosynthetic device will consist ofa transparent enclosed slurry reactor with suspended PAHs. The PAHconsists of a light absorbing semiconductor junction polarized by acathode electrocatalyst and an anode electrocatalyst with thesemiconductor protected by a non-corrosive insulating layer (See FIG.7A-7B). One or both of the electrocatalysts will have a semipermeablecoating to prevent any back reactions (e.g. chromium oxide which willallow H⁺ and H₂ but not organics to the cathode). Under illumination,the PAHs act as light harvesting antennae absorbing sunlight andproducing usable charges which are then driven to the external contactsto carry out the necessary redox reactions producing valuable fuels andchemicals (See FIGS. 8A-8D). In one embodiment, the coating material asdescribed in U.S. Provisional Application No. Application No.61/559,717, filed Nov. 14, 2011, the disclosure of which is incorporatedherein by reference, can be any conducting polymer which is opticallytransparent, electrically conducting, electrocatalytically active,energetically forming a type-II band offset with an underlyingsemiconductor layer, and stable in acidic and basic electrolytes. Insuch embodiments, coating materials include, but are not limited to,Poly (3, 4-ethylenedioxythiophene) (PEDOT) in natural and doped form,Poly (4, 4-dioctylcyclopentadithiophene) in natural and doped form,metallic carbon nanotubes (CNTs), or combinations thereof. The dopantsfor PEDOT include, but are not limited to, polystyrene sulfonate (PSS),tetra methacrylate (TMA), or combinations thereof. The dopants for poly(4, 4-dioctylcyclopentadithiophene) include, but are not limited to,iodine, 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ), andcombinations thereof.

More specifically, in some embodiments, processes for developing stableartificial PS device comprise the steps: a) selecting/fabricating a highquality semiconductor such as silicon, gallium arsenide, indiumphosphide, cadmium selenide, cadmium telluride, copper zinc tin sulfide,copper sulfide, tin sulfide, iron sulfide etc.; b) dispersing thecoating material in a mixture of aqueous/non-aqueous solvent andhomogenizing them; c) disposing the coating material on top of theabsorber layer; d) annealing the coating to form a Schottky contact orto form an efficient hole transport layer depending upon the choice anddesign of the underlying semiconductor layer; d) positioning ofelectrocatalyst on the back of the absorber layer followed by annealingto form an Ohmic contact; e) mechanical breaking of the entire unitusing ball mill, dicing saw or by laser cutting to obtain individualmicron size PS units; and f) suspending the unit in a electrolyte (SeeFIGS. 9A-9C). In one example, the light absorption occurs in the p-nGaAs photovoltaic structure. The wireless PS cell was fabricated bye-beam depositing platinum at the cathode side (n-side) and spin castingPEDOT: PSS on the anode side (p-side) of GaAs wafer and was suspended infuming hydrobromic acid (HBr) for production of H₂ and Br₂. PEDOT: PSSserved as both transparent conducting hole transport layer and as anelectrocatalyst for bromine evolution. Upon illumination, clear visualobservation of H₂ bubbles on the cathode and bromine evolution at theanode was obtained. With no PEDOT: PSS coating, the cells failedimmediately in HBr. The stability of the artificial photosynthetic unitwas assessed by measuring the hydrogen production using a gaschromatograph (GC) column in a closed cell configuration. The cell wasstable for 6 hours of continuous operation.

In another embodiment of the present invention the above PAH unit wasused for oxidation of organic wastes to produce clean water and H₂.

In another embodiment of the present invention the above PAH unit wasused for direct water splitting to produce O₂ and H₂

In another embodiment the above PAH unit was used for oxidation offormic acid to produce CO₂ and H₂

In another embodiment of the present invention, the oxidation andreduction electrocatalysts were loaded using photochemical processes.For example, gallium arsenide wafer can be first used to photoreduce(using chloroplatinic acid) Pt onto positions where electrons willtransport, with the counter reaction being a facile non-metal depositionstep (such as methanol oxidation), then in a separate solution such asnickel acetate and a sacrificial electron acceptor expose the solutionto light to form an anode, such as Ni0OH through an oxidation reaction,so that the Pt has been deposited where electrons transport and theNiOOH (which when oxidized will form NiO) will be deposited where theholes will transport.

EXAMPLE 2 An Exemplary Stable Artificial Photoelectrosynthetic (PS)Device Based on Porous Anodic Aluminum Oxide (AAO) membrane forProduction of Fuels and Chemicals

This example serves to exemplify the demonstration of fuel and chemicalproduction in a device structure based on a porous AAO with low band gapsemiconductor materials deposited within the pores and capped with anodeand cathode electrocatalysts. One or both of the electrocatalysts willhave a semi permeable coating to prevent any back reactions (e.g.chromium oxide which will allow H⁺ and H₂ but not organics to thecathode).

The porous AAO serves as a protective membrane to solve the issue ofinstability of low band gap semiconductors when dipped in commonelectrolytes. Anode and cathode electrocatalysts, such as transitionmetals, caps the top and bottom of the semiconductor so as to separatethe semiconductors from the electrolyte, reducing the chance ofcorrosion. An additional functional coating on the cathode will allowdiffusion of protons and hydrogen but block the oxidation products fromreacting and thus increase the Faradaic efficiency for production ofhydrogen.

The process begins by creating a uniform array of elongatedsemiconductor absorber materials in the range 50-1000 nm in length andwith diameters ranging from 20-200 nm by electrodeposition inside aproperly fabricated porous anodic aluminum oxide (AAO) template. PorousAAO template is fabricated my methods as reported by us and elsewhere(step 1). The process begins by electropolishing aluminum foil in 1:5mixtures of perchloric acid and ethanol at 20V in 5° C. Theelectropolished aluminum foil is then electrochemically anodized in 0.3M oxalic acid or sulfuric acid. The anodizing voltages ranges from 20 Vto 120 V depending on the required pore diameter and pore density. Thepores may be further widened to desired diameter as reported elsewhere.The anodized aluminum oxide is then removed from the aluminum underlayerand the remaining alumina barrier layer is removed by wet or dry etchingstep (step 2). A thin metallic film is physical vapor deposited on oneof the sides of porous AAO layer to form an electrically conductingbacking layer (step 3). The material selected for the backing layer ischosen such a way that it can be mechanically or chemically etched lateron. In step 4, one deposits the necessary PV device with appropriateelectrocatalysts into the pores using electrodeposition or vapor-baseddeposition techniques (chemical vapor deposition, atomic layerdeposition, etc.) followed by removal of the backing metallic film (step5). For example, FIG. 16 illustrates such a device 1200 showing ahoneycomb-shaped protective structure 1202 housing isolated andautonomous light absorbing units 1204, which each have isolated anodes1206 and isolated cathodes 1208. In one embodiment the absorbermaterials include low band gap materials such as CdTe, Si, GaAs, InP andmetal sulfides CdS, Cu₂S, SnS, Cu₂ZnSnS₄, etc. To form an electric fieldfor charge separation, diode junctions (such as CdS/CdTe), or Schottkyjunctions (such as CdSe/PEDOT: PSS) are fabricated inside the pores. Theresult is a dense array of nanometer sized PAHs separated from eachother by a transparent protecting alumina membrane, and each unit withinthe pores serving as an autonomous solar fuel production unit maximizingthe fault tolerance.

In one embodiment, a hybrid CdSe-PEDOT nanowire based PAH unit isfabricated with CdSe as the light absorber and PEDOT: PSS as the holefilter and oxidation catalyst. Each nanowire PAH unit consists of ann-CdSe light absorbing layer connected in series to a platinumelectrocatalyst through a nickel-gold Ohmic contact segment. The otherend of the CdSe is connected to PEDOT which functions both as a holetransporting Schottky contact layer and also as the anodeelectrocatalyst. The PAH unit was suspended in acidified HI solution.Upon illumination, holes flow towards the Schottky contact producing I₂and electron flows toward the Ohmic contact to produce H₂. Only thecathode (Pt) and anode (PEDOT) electrocatalysts is exposed to thesolution and CdSe is protected by the AAO.

In one embodiment of the present invention, a hybrid CdSe-PEDOT nanowirebased PAH unit was used for formic acid oxidation to produce CO₂ and H₂.

In another embodiment of the present invention, a hybrid CdSe-PEDOTnanowire based PAH unit was used for methanol oxidation.

In another embodiment of the present invention, Cu/Cu2O/Au Schottkybarrier solar cell was fabricated inside PAO for formic acid oxidationstudies.

EXAMPLE 3 Photoelectrochemically Active Heterostructures using Siliconas the Photoabsorber

In another embodiment of the invention silicon coated with PEDOT to forma Schottky junction was used as a photoelectrosynthetically activeheterostructure to produce H₂ with the counter reactor the oxidation ofvanadia ions.

In another embodiment the silicon coated with PEDOT to form a Schottkyjunction was used to produce H₂ and I₂, as shown in FIGS. 12A-12B.

In another embodiment of the present invention the device describedabove could be used for the production of H₂ and the oxidation ofmethanol.

EXAMPLE 4 Photoelectrochemical Reactor System for Production of Hydrogen

An embodiment of the present invention consists of a reactor thatcontains electrolyte, is permeable to sunlight, and is impermeable tothe products, with a method of extraction for the products withoutdestruction of the bag or need of removal of the electrolyte, and thatcontains the photoelectrochemically active heterostructures immersed andsuspended in the electrolyte.

The reactor may consist of any plastic that is impermeable to theelectrolyte and the gases (for example hydrogen and oxygen) such as aFood Saver baggie where the ends are sealed using a heat sealing method,or a Tedlar gas bag that has been sealed.

In order to maintain permeability to sunlight, the plastic material canbe selected that will maintain permeability to light over time, as wellas defogging agents that can be placed on the reactor to stopcondensation of water vapor on the baggie surface, which can block thepermeability of light into the reactor system. One example of avoidingthis issue is using an anti-fogging material such as Rain-X in order toavoid the condensation of water that stops light from penetrating thebaggie, shown in FIGS. 13A and 13B.

In order to suspend the PAH in the electrolyte, natural convection dueto temperature gradients can be used to circulate the electrolyte. Onemethod of using this is a checkerboard black/white pattern on the bottomof the baggie so that light that is not absorbed by the PAH structuresmay be preferentially absorbed on the black portions of the bottom ofthe baggie, causing differential heating between the black and whiteportions of the pattern, which can lead to natural convection and amixing of the solution without need of an external stirring source. Anexample of such a pattern is shown in FIG. 14.

The gas that is formed in the headspace of the baggie can be collectedthrough an opening in the same manner as a gas-bag that is commerciallyavailable, and brought to any location where the H₂ is required, such asa fuel cell.

EXAMPLE 5 Reducing the Rate of Backreaction in PhotoelectrosyntheticSolar Energy Conversion

In one embodiment of the invention the backreaction may be preventedthrough the use of a protective coating layer. The protective coatinglayer may consist of, but is not limited to, Cr₂O₃, or a polymer thatpreferentially allows H+ to pass through while blocking the ability ofthe oxidation product (ex. Br₂/Br₃ ⁻) to pass through. An example ofsuch a protective coating is a thin coating of Nafion, which is anexcellent membrane for blocking bromine to prevent the parasiticbackreaction of bromine reduction, which reduces the faradaic efficiencyof hydrogen production, during photoelectrochemical electrolysis of HBr.Nafion preferentially allows H⁺ to pass through, while minimizing theamount of bromine that can pass through. A comparison of the effect ofNafion in reducing the bromine oxidation (seen by the reduction currentat higher potentials than the electrochemical reduction of water)compared to an uncoated and PEDOT coated electrode as well as a Tefloncoated Pt electrocatalyst is shown in FIG. 15A. In addition to theeffect of the reduction in backreaction, and increase in Faradaicefficiency of the production of products, the Nafion also serves toprotect the hydrogen electrocatalyst from the presence of corrosivespecies such as bromine or bromide, which may otherwise poison orcorrode the platinum electrocatalyst.

In another embodiment of the invention, the backreaction may beprevented by the complexation of the products to decrease theireffective concentration in the solution. Polyethylene glycol can be usedto complex bromine and reduce the backreaction of bromine reduction, asshown in FIG. 15B.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method of manufacturing aphotoelectrosynthetically active heterostructure, comprising: forming orproviding one or more cavities in an electrically insulating material;forming or providing a layer of an electrically conductive metal on oneside of the electrically insulating material; depositing anelectrocatalyst cathode layer in the one or more cavities; depositingone or more layers of light-absorbing semiconductor material in the oneor more cavities; depositing an electrocatalyst anode layer in the oneor more cavities; removing the layer of electrically conductive metal;and forming a hydrogen permeable layer over the electrocatalyst cathodelayer.
 2. The method of claim 1, wherein the one or more layers oflight-absorbing semiconductor material comprise one or more p-njunctions formed by one or more layers of p-type semiconductor materialadjacent to one or more layers of n-type semiconductor material.
 3. Themethod of claim 1, wherein the one or more layers of light-absorbingsemiconductor material comprise one or more layers of p-typesemiconductor material or n-type semiconductor material that form one ormore Schottky junctions.
 4. The method of claim 1, wherein theelectrically insulating material includes an array of nanosized cavitiesor pores so that the photoelectrosynthetically active heterostructurecomprises a plurality of independent nanosized light absorbing unitsdisposed within the array of cavities or pores.
 5. The method of claim4, wherein the electrocatalyst anode layer is coupled to the independentlight absorbing units and isolated from the one or more cathodes so thateach independent light absorbing unit is autonomous from other lightabsorbing units.
 6. The method of claim 1, wherein the electricallyinsulating material is selected from the group consisting of Al₂O₃,SiO₂, ZrO, AlF₃, TiF₂, ZnO, TiO₂, metal oxides, and oxides of n-type orp-type semiconductor materials.
 7. The method of claim 1, whereinforming or providing the one or more cavities within an electricallyinsulating material comprises forming a plurality of cavities within ametal foil and oxidizing metal surfaces within the cavities to form anelectrically insulating metal oxide.
 8. The method of claim 7, whereinthe electrically insulating metal oxide is formed using at least one ofdry etching or electrochemical anodization.
 9. The method of claim 1,wherein the electrocatalyst cathode layer, the light-absorbingsemiconductor material, and the electrocatalyst anode layer aredeposited in the one or more cavities by one or more ofelectrodeposition, vapor deposition, chemical vapor deposition, oratomic layer vapor deposition.
 10. The method of claim 1, wherein thelayer of electrically conductive metal is removed by mechanical orchemical etching.
 11. The method of claim 1, wherein the electrocatalystcathode layer and/or the electrocatalyst anode layer are deposited inthe one or more cavities prior to removing the layer of electricallyconductive metal.
 12. The method of claim 1, wherein the electrocatalystcathode layer and/or the electrocatalyst anode layer are deposited inthe one or more cavities after removing the layer of electricallyconductive metal.
 13. The method of claim 1, wherein the one or morelayers of semiconductor material comprise at least one p-typesemiconductor material selected from the group consisting of intrinsicor p-doped SnS, ZnS, CdS, CdSe, CdTe, Cu₂S, WS₂, Cu_(x)O, Cu₂ZnSnS₄,CuIn_(x)Ga_(1-x)Se₂, GaN, InP, or SiC; doped (p-type) or undopedelemental Si or Ge; and doped (p-type) or undoped compoundsemiconductors selected from metal sulfides, selenides, arsenides,nitrides, antinomides, phosphides, oxides, tellurides, and mixturescontaining, respectively, sulfur (S) selenium (Se), arsenic (As),antimony (Sb), nitrogen (N), oxygen (O) tellurium (Te), and/orphosphorus (P) as one or more electronegative element(s) A, and one ormore metals (M), of the form M_(n)A_(x) where M is one or a combinationof elements selected from Cu, Ga, Ge, Si, Zn, Sn, W, In, Ni, Fe, Mo, Bi,Sb, or Mg.
 14. The method of claim 1, wherein the one or more layers ofsemiconductor material comprise at least one n-type semiconductormaterial selected from the group consisting of intrinsic or n-doped InS,CdTe, CdS, CdSe, CdTe, Cu₂S, WS₂, Cu_(x)O, Cu₂ZnSnS₄,CuIn_(x)Ga_(1-x)Se₂, GaN, InP, or SiC; doped (n-type) or undopedelemental Si or Ge; and doped (n-type) or undoped compoundsemiconductors selected from metal sulfides, selenides, arsenides,nitrides, antinomides, phosphides, oxides, tellurides, and theirmixtures containing respectively, sulfur (S), selenium (Se), arsenic(As), antimony (Sb), nitrogen (N), oxygen (O), tellurium (Te), and/orphosphorus (P) as one or more electronegative element(s) (A), and one ormore metals (M), of the form M_(n)A_(x) where M is one or a combinationof elements selected from Cu, Ga, Ge, Si, Zn, Sn, W, In, Ni, Fe, Mo, Bi,Sb, or Mg.
 15. The method of claim 1, wherein the electrocatalystcathode layer comprises at least one conductor material selected fromthe group consisting of platinum group metals, Pt, Au, transitionmetals, transition metal oxides, NiO, metal carbides, WC, metalsulfides, MoS₂, electrical conducting carbon containing materials,graphite, graphene, and carbon nanotubes.
 16. The method of claim 1,wherein the electrocatalyst anode layer comprises at least one conductormaterial selected from the group consisting of metals, oxides andmixtures of metals selected from Ru, Ag, V, W, Fe, Ni, Pt, Pd, Ir, Cr,Mn, Cu, or Ti; metal sulfides; MoS₂; electrical conducting carboncontaining materials; graphite; graphene; and carbon nanotubes.
 17. Themethod of claim 1, wherein the hydrogen permeable layer comprises atleast one material selected from the group consisting of chromium (III)oxide (Cr₂O₃), nafion membranes made from sulfonatedtetrafluoroethylene-based fluoropolymer-copolymers, acrylics, mixturesof metal oxides, WCrO_(x), and WZrCeO_(x).
 18. A method of manufacturinga photoelectrosynthetically active heterostructure, comprising: formingor providing a protective membrane comprising an electrically insulatingsheet and an array of cavities in the electrically insulating sheet;forming or providing an electrically conductive metal layer on one sideof the electrically insulating sheet; depositing by electrodeposition,vapor deposition, chemical vapor deposition, or atomic layer vapordeposition an electrocatalyst cathode layer in the array of cavities;depositing by electrodeposition, vapor deposition, chemical vapordeposition, or atomic layer vapor deposition one or more layers oflight-absorbing semiconductor material in the array of cavities;depositing by electrodeposition, vapor deposition, chemical vapordeposition, or atomic layer vapor deposition an electrocatalyst anodelayer in the array of cavities; and removing the electrically conductivemetal layer from the electrically insulating sheet.
 19. The method ofclaim 17, further comprising forming a hydrogen permeable layer over theelectrocatalyst cathode layer.
 20. A method of manufacturing aphotoelectrosynthetically active heterostructure, comprising: forming orproviding a protective membrane comprising anodized aluminum oxide andan array of nanosized pores in the protective membrane; forming orproviding an electrically conductive metal layer on one side of theprotective membrane; electrodepositing an electrocatalyst cathodematerial in the array of nanosized pores; electrodepositing one or morelayers of light-absorbing semiconductor material in the array ofnanosized pores; electrodepositing an electrocatalyst anode material inthe array of nanosized pores; and removing the electrically conductivemetal layer from the protective membrane.